A New Horizons spacecraft Long Range Reconnaissance Imager (LORRI) image of Jupiter and two of its moons, taken on June 24 when the spacecraft was 2.4 billion kilometres from the giant planet.

New Horizons spacecraft bound for Pluto

Has taken images of Jupiter and Neptune

Pluto rendezvous set for July 14, 2015

NASA’s Pluto-bound spacecraft, New Horizons—which in December 2009 passed the halfway mark in its 10-year journey to the distant world—has turned around to take a look back into the middle part of the Solar System.

On June 24, the spacecraft’s Long Range Reconnaissance Imager (LORRI) was trained on Jupiter, the largest planet in the Solar System.

Artist's impression of how the New Horizons spacecraft will look during its fly-by of Pluto in July 2015.

A little over three years ago, New Horizons made a close fly-by of Jupiter to pick up speed using a “gravitational slingshot”. Now, in July 2010, the spacecraft is 2.4 billion kilometres from the planet—1,000 times further than it was at the moment of closest approach during that fly-by.

“The picture is a dramatic reminder of just how far New Horizons, moving about a million miles [1.6 million kilometres] a day, has travelled,” says mission Principal Investigator Alan Stern, of the Southwest Research Institute.

Despite the huge distance, the disc of Jupiter is readily apparent due to the planet’s large size. And because from LORRI’s perspective there was a good angle between Jupiter and the Sun, the planet looks a bit like a half full Moon.

And speaking of moons, two of Jupiter’s can faintly be seen—Ganymede on the left and Europa on the right.

Calibration image

The main aim of the Jupiter image was to test LORRI’s susceptibility to sunlight. LORRI is designed to operate in the distant, pitch black environment of Pluto, where sunlight is hundreds of times dimmer than it is here on Earth. So the camera is very sensitive; too much light would damage it. Which is why the Jupiter image was made with an exposure of only 0.009 second, and why the two moons appear so faint.

“We wanted to see how much stray sunlight would creep into these Jupiter pictures, especially since we’ll make observations of the Pluto system in a similar geometry after the spacecraft passes Pluto in 2015,” says Project Scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory.

“We generally prefer to look at targets in the opposite direction from the Sun. In fact, LORRI is calibrated for the low light we’ll see in the Pluto system and Kuiper belt. Pointing too close to the Sun could damage the camera, but we decided it was safe to try to observe Jupiter.”

“The observations were successfully executed and the images look great.”

Neptune as seen by New Horizons from a distance of around 3.4 billion kilometres.

A glance at Neptune

LORRI also was pointed toward another of the Solar System’s planets, Neptune. The huge blue world is around six times further from the Sun than Jupiter, so in the LORRI image it looks just like a fuzzy star.

The 100-millisecond exposure, made on June 23 when New Horizons was still 23 astronomical units from Neptune (one astronomical unit is the distance between the Sun and the Earth), was part of a navigation system test. New Horizons is equipped with “optical navigation”, a fancy term that means trajectory corrections during the final stages of its approach to Pluto will be aided by taking pictures of the planet and comparing its apparent position with its calculated position.

Even though no detail can be made out on Neptune’s disc, scientists can still use the images to learn more about the planet’s atmosphere, by seeing how sunlight scatters off the molecules.

On course for Pluto

At the beginning of July, the New Horizons team instructed the spacecraft to make a 35.6-second burn of its thruster. Calculations had shown the craft drifting slightly off course, due to a tiny amount of force applied by heat coming from the spacecraft’s radioisotope power source and reflecting off the main antenna!

With the burn correctly executed, New Horizons is on target for its closest approach to Pluto at 7:49am US EDT on July 14, 2005.

Due to mass constraints, the craft is not equipped with a braking rocket, so it will not be able to stop. Instead, it will go sailing straight past the tiny dwarf planet at a great rate of knots, and then head out into deep space. Mission planners are hoping they can find one or more candidate Kuiper Belt Objects—the small, icy worlds that inhabit the outer Solar System—and steer New Horizons to another rendezvous.

Our very best views of Pluto's surface, made from multiple Hubble Space Telescope images taken in 2002-03. From 200 days out, New Horizons will begin to take better photos than these.

Even though New Horizons will be unable to stop at Pluto, that doesn’t mean it won’t get a good look at the small planet and its three known moons. From roughly 200 days before the encounter right through until many days after, the spacecraft will take images that are better than the Hubble Space Telescope can produce.

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